1. Field.
The present invention relates to a system in which a residual vacuum condition is employed for maintaining an electrical accessory circuit of an internal combustion engine in an energized condition for a selected time interval after the engine is rendered inoperative.
2. Prior Art.
In the prior art it is known to employ a residual vacuum for the purpose of maintaining an electrical switch in a selected condition. Such systems often employ an evacuated chamber connected to a flow restrictor for bleeding the evacuated chamber to atmosphere. In such cases, the time interval is determined by the restriction of the flow restrictor, the volume of the evacuated chamber, and the level of subatmospheric depression in the chamber. Where the intake manifold of an internal combustion engine is employed for evacuating the chamber, the subatmospheric depression in the chamber is apt to vary, for example, depending upon whether the engine was revved up or allowed to idle immediately before the ignition switch was turned off. Such variations in the level of subatmospheric depression may result in variations in the time interval. Improvements in vacuum operated timing systems are desirable in order to provide a repeatably uniform time delay for accessory circuits of internal combustion engines.